Hydrogen is commonly used in various industrial applications. For example, in a fuel cell, hydrogen gas is used as a fuel to generate electrical power which operates a vehicle or other machinery. The hydrogen is typically stored in liquid form in a hydrogen tank and distributed from the tank to the fuel cell for use.
In liquid hydrogen tanks, the hydrogen typically exists in both a liquid portion and a gaseous portion. The temperature is maintained at a very low temperature (typically about −250° C.). The hydrogen may typically be distributed from the tank for use in a fuel cell, for example, in either gas or liquid form. Although some systems provide for the distribution of exclusively liquid or gaseous hydrogen from the tank, it may be desirable to shift between distributing either gaseous or liquid hydrogen from the tank depending on the tank conditions.
During the distribution of gaseous hydrogen from the tank, the pressure in the tank has a tendency to decrease at such a high rate that flow of hydrogen from the tank stalls. This phenomenon has been solved by providing a heater in thermal contact with the tank. In that case, heat transferred into the tank from the heater evaporates some of the liquid hydrogen into the gaseous phase, thereby increasing the tank pressure. Adding a heater to the tank, however, imparts additional complexity to the system.
During the distribution of liquid hydrogen from the tank, the tank pressure has a tendency to remain more stable than is the case with respect to gaseous hydrogen. Natural transfer of heat from extraneous heat sources into the tank, however, has a tendency to raise the temperature of the hydrogen in the tank. If the flow rate of liquid hydrogen from the tank is low, the raised temperature can lead to excessively high pressure levels in the tank.
Therefore, it is desirable to distribute liquid hydrogen from a hydrogen storage tank at high flow rates and/or low tank pressures and to distribute gaseous hydrogen from the tank at low flow rates and/or high tank pressures.